A normal brushless permanent magnet ironless rotating motor or other kind of rotating motor can only deliver torque, i.e. perform a rotating movement. However, in a lot of applications a rotating and translating movement is required. Usually, such rotating and translating movement is made by two motors or a very specific designed motor with complex electronics and control techniques.
U.S. Pat. No. 6,429,611 B1 discloses a combination rotary and linear motor which uses a modified brushless direct current (DC) motor. The motor comprises, among other things, a rotor, a motor casing, and three radially equi-spaced coils mounted to the interior cylindrical surface of the motor casing. An angular position θ of the rotor and a linear position z thereof along its rotation axis (z-axis) are controlled in accordance with command values θc and zc, respectively. This is achieved by driving the coils accordingly. Currents Ia, Ib and Ic supplied to the coils are determined by means of a feedback loop comprising inter alia current sensors for sensing the currents Ia and Ib as well as proportional-integral (PI) controllers.
U.S. Pat. No. 6,429,611 B1 does not disclose an explicit formula for the currents Ia, Ib and Ic supplied to the coils. The currents Ia and Ib are derived by means of a current feedback. Therefore, they have to be sensed by current sensors. Furthermore, PI controllers are needed to implement the feedback loop. These PI controllers are dynamic elements and have to be tuned to make them work. That is, the PI parameters have to be tuned. Thus, a certain amount of electronic and/or software components is needed to operate the combination rotary and linear motor described in U.S. Pat. No. 6,429,611 B1.